Laser having means for enhancing the tem{11 {11 mode output thereof

ABSTRACT

A transverse acoustic wave is applied to a fused silica cell disposed within a laser cavity to thereby change the optical path length of the laser cavity. TEMoo mode efficiency is improved by increasing the number of atoms in the active laser medium which contribute to the axial mode.

[161 3,694,768 1451 Sept. 26, 1972 Unllefl DIa Young et al.

[54] LASER HAVING MEANS FOR ENHANCING THE TEM MODE OUTPUT THEREOF [72]Inventors: Donald S. Young, Windham; William C. Fricke, Milford, both ofNJ-i.

[73] Assignee: Sanders Associates, Inc., Nashua,

Nil.

[22] Filed: Oct. 12,1970

21 Appl.No.: 80,090

[52] U.S.Cl. ..33l/94.5, 250/199, 350/161 [51] Int. Cl ..H0ls 3/10 [58]Field ofSearch ..33l/94.5; 280/199; 350/161 [56] References Cited UNITEDSTATES PATENTS 3,297,876 1/1967 De Maria ..250/l99 3,412,251 11/1968l-largrovem, ..2so/199 OTHER PUBLICATlONS Danielmeyer et al., AppliedPhysics Lett. 16, 1970, p. 124

Rabson et al., IEEE Annual SW Conf. & Exhib. 22d, Dallas, Texas April22- 24, 1970, pp. 536- 40.

Primary Examiner-Ronald L. Wibert Assistant Examiner-R. J. WebsterAttorney-Louis Etlinger [57] ABSTRACT A transverse acoustic wave isapplied to a fused silica cell disposed within a laser cavity to therebychange the optical path length of the laser cavity. TEM, mode efficiencyis improved by increasing the number of atoms in the active laser mediumwhich contribute to the axial mode.

9 Claims, 1 Drawing Figure OSCILLATOR PUMP POWER -14 1 SUPPLY 24 0Rmast/945g:

PATENTEHSEPZB m2 3.694.768

OSCILLATOR PUMP I POWER -|4 l SUPPLY 24 //V 5 N TORS DONALD S. YOUNGWILLIAM C. FRICKE AGENT BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates generally to the field of lasersand more particularly to apparatus for enhancing the TEM. mode output oflasers.

2. Description of Prior Art The resonant cavity of a conventional laseris of very high order, i.e. the cavity dimensions are very much largerthan the emitted radiation wavelength. Such lasers are thus multimodedevices in that there are a number of frequencies which correspond tothe various longitudinal and transverse oscillation modes. Prior to thepresent invention the generation of TEM longitudinal modes in a laserusually involved placing an aperture in the resonant cavity to provide ahigh loss for all but the TEM mode in addition to an etalon to obtain asingle frequency. Although this approach may provide an improvement inthe mode purity of the laser output radiation it also involved a loss of80-90 percent of the CW power available. This loss resulted from the Iprevention of atoms in the homogeneously broadened laser transition atthe nodal planes of the axial modes from contributing to the off axismodes. If these atoms may be made to contribute to the axial mode thenhigher TEM mode efficiencies may be achieved. One method of providingthis capacity was to physically move the active laser medium relative tothe resonant cavity by suspending the medium on an air cushion andoptical bench. This method, however, provided movement which could onlybe maintained for a period of about ten milliseconds and presented verydifi'icult problems in aligning the active medium with the resonantcavity reflectors.

OBJECTS AND SUMMARY OF THE INVENTION From the foregoing discussion itwill be understood that among the various objectives of the presentinvention are:

To provide a new and novel apparatus for enhancing the TEM mode outputof a laser.

To provide apparatus of the above described character having norequirement for moving parts.

To provide apparatus of the above described character which introducesminimal degradation of the laser power output.

To provide apparatus of the above described character wherein theresonant cavity is shifted relative to the atoms of the active lasermedium.

To provide apparatus of the above described character using anacousto-optic cell in the laser resonant cavity.

These and other objectives of the present invention are efficientlyattained by providing an acousto-optic cell within the laser resonantcavity. A transverse acoustic standing wave is set up in the cell whichoperates to change the optical path length of the resonant cavity at apreselected rate and thereby increase the number of atoms in the activelaser medium which contribute to the axial oscillation mode thusincreasing the TEM mode efficiency.

The foregoing as well as other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription taken in conjunction with the appended drawing.

2 BRIEF DESCRIPTION OF THE DRAWING The sole appended FIGURE is aschematic view of a laser incorporating the improvement according to thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENT Turning now to the appended drawingthere is illustrated an active laser medium 10 such as for example anyttrium-aluminum-garnet (YAG) rod. A pump lamp 12 driven by aconventional power source 14 is illustrated schematically as beinghelically disposed about the active medium 10. The active medium 10 isdisposed along the longitudinal axis 16 of a resonant cavity defined bya totally reflective mirror 18 and a partially reflective output mirror20. Also disposed within the resonant cavity is an acousto-optic cell 22such as a quartz block which is transparent to the output radiationwavelength of the laser. An oscillator 24 is coupled via a piezoelectrictransducer 26 to the acousto-optic cell 22. An aperture plate 28 is alsodisposed in the cavity between the electro-acoustic cell 22 and theactive laser medium 10 with the aperture disposed on the longitudinalaxis 16.

According to the principles of the present invention the optical pathlength of the resonant cavity is changed at a preselected rate bychanging the index of refraction of the acousto-optic cell 22. Toaccomplish this a transverse acoustic standing wave schematicallyillustrated at 30 is set up in the acousto-optic cell 22.

- The transverse dimension of the acousto-optic cell 22 thus must beequal to a half wavelength of the acoustic wave. The frequency of theacoustic wave and thus the dimension of the cell 22 is selected suchthat it is large enough to generate the desired rate of change in theoptical path length but low enough that the index of refraction of thecell 22 is substantially constant at any instant in time across thediameter of the laser beam.

By way of illustration, one embodiment of the present invention includesa one quarter inch by 3 inch cylindrical neodymium doped YAG laser rodand a crystalline quartz acousto-optic cell having a transversedimension of approximately 5 centimeters and a thickness of 4centimeters. The frequency of the oscillator signal applied to theelectro-acoustic cell is 57 KHz which produced a transverse acousticstanding wave having a wavelength of 10 centimeters. The transverse modeis preferred in the practice of the present invention in order that thequartz cell does not become stress birefringent and thus introduce highlosses. The laser beam diameter was 0.4 centimeter and the index ofrefraction of the electro-acoustic cell was found to be substantiallyconstant over 96 percent of the beam diameter. The rate of change inoptical path length provided by this apparatus was about 1 centimeterper second which was sufficient to provide an improvement in TEM modeefficiency of more than 50 percent. It was found that this arrangementproduced a substantial elimination of spatial hole burning at theanti-nodal planes of the TEM mode laser.

It will thus be apparent that the applicants have provided apparatuswhich yields a significant improvement in TEM mode efficiency over thatavailable in the prior art. Since certain changes in the above describedconstruction will become apparent to those skilled in the art withoutdeparting from the scope of the invention it is intended that all mattercontained in the foregoing discussion or shown in the appendedillustration shall be taken as illustrative and not in a limiting sense.

Having described what is new and novel and desired to secure by LettersPatent, what is claimed is:

1. An improved laser including a resonant optical cavity, an activelaser medium axially disposed within said cavity, means for pumping saidactive laser medium such that the emission of a beam of radiationtherefrom is stimulated which beam oscillates within said resonantcavity, an aperture plate disposed within said cavity and having theaperture disposed on the longitudinal axis thereof for eliminatingoff-axis modes, and wherein said improvement comprises an acoustic-opticcell axially disposed within said resonant cavity, and

means for producing a transverse acoustic standing wave in saidacousto-optic cell to vary the index of refraction thereof at apreselected rate thereby providing a preselected rate of variation inthe effective length of said resonant cavity such that a substantialelimination of spatial hole burning at the antinodal planes of the laseroccurs, said acoustic wave having a frequency high enough such as togenerate the desired rate of change in the optical path length but lowenough such that the index of refraction of said cell is substantiallyconstant at any instant in time across the diameter of the laser beam.

2. Apparatus as recited in claim 1 wherein said acousto-optic cell isformed of fused silica.

3. Apparatus as recited in claim 1 wherein said acousto-optic cell isformed of fused silica.

4. Apparatus as recited in claim 1 wherein the transverse dimension ofsaid acousto-optic cell is substantially equal to a half-wavelength ofsaid acoustic wave.

5. Apparatus as recited in claim 1 wherein the frequency of saidacoustic wave is sufficiently low that the index of refraction issubstantially constant across the diameter of said beam of radiation.

6. Apparatus as recited in claim 1 wherein the rate of change of saideffective length of said resonant cavity is at least 1 centimeter persecond.

7. Apparatus as recited in claim 1 wherein said active laser medium is aneodymium doped yttrium-aluminum garnet rod,

said acousto-optic cell is a crystalline quartz block having atransverse dimension of 5 centimeters, and

said acoustic wave producing means includes an oscillator coupled tosaid acousto-optic cell and having an output frequency of substantially57 kilohertz.

8. Apparatus as recited in claim 7 wherein said acoustic wave is in thetransverse mode.

9. Apparatus as recited in claim 7 wherein the diameter of said beam ofradiation from said rod is substantially 0.4 centimeter,

said rate of change of said effective length of said resonant cavity issubstantially 1 centimeter per se 0nd, and the index of refraction ofsaid crystalline quartz block is substantially constant over at least0.38 centimeter.

2. Apparatus as recited in claim 1 wherein said acousto-optic cell isformed of fused silica.
 3. Apparatus as recited in claim 1 wherein saidacousto-optic cell is formed of fused silica.
 4. Apparatus as recited inclaim 1 wherein the transverse dimension of said acousto-optic cell issubstantially equal to a half-wavelength of said acoustic wave. 5.Apparatus as recited in claim 1 wherein the frequency of said acousticwave is sufficiently low that the index of refraction is substantiallyconstant across the diameter of said beam of radiation.
 6. Apparatus asrecited in claim 1 wherein the rate of change of said effective lengthof said resonant cavity is at least 1 centimeter per second. 7.Apparatus as recited in claim 1 wherein said active laser medium is aneodymium doped yttrium-aluminum garnet rod, said acousto-optic cell isa crystalline quartz block having a transverse dimension of 5centimeters, and said acoustic wave producing means includes anoscillator coupled to said acousto-optic cell and having an outputfrequency of substantially 57 kilohertz.
 8. Apparatus as recited inclaim 7 wherein said acoustic wave is in the transverse mode. 9.Apparatus as recited in claim 7 wherein the diameter of said beam ofradiation from said rod is substantially 0.4 centimeter, said rate ofchange of said effective length of said resonant cavity is substantially1 centimeter per second, and the index of refraction of said crystallinequartz block is substantially constant over at least 0.38 centimeter.